412 E. J. ALLEN AND E. AV. NELSON. 



frustiiles of Thalassiosira could he found, whilst tlie 

 growth of Pleurococcus was abundant and healthy. The 

 only case where a diatom was observed to flourish in the 

 presence of this green alga was in a culture of Nitzschia, 

 a bottom form. In this case a very abundant growth of the 

 diatom was obtained, but the Pleurococcus did not 

 multiply to any extent although it could always be found on 

 microscopic examination. 



III. Notes on Particular Species of Diatoms, on their 

 Methods oi-' Reproduction, and on other Alom Occur- 

 KiNG IN Cultures. 



A list has been already given (p. 367) of those species of 

 diatoms which we have obtained in '' jiersisfent " cultures. 

 Of these a species belonging to the genus Thalassiosira 

 has been used for experimental work in the great majority of 

 cases. AYe are not quite certain as to the identity of the 

 species, but since it most resembles T. decipiens Grun, we 

 have called it by that name, although it does not exactly con- 

 form to the published descriptions of that form. The most 

 characteristic feature of this particular species is the eccentric 

 markings on the valves, which are also seen on the valves of 

 the diatom Coscinodiscus excentricus Ehr., and, as is 

 typical of the genus, the frustules ai'e united into chains by 

 a delicate filament. Jcirgensen (50, p. 96) describes the valves 

 as "decidedly convex," Gran (49) as "flat," and both agree 

 that there are marginal spines and a single asymmetrical 

 spine. Our cultural forms are united together by a filament 

 into chains, some of which are made up of 500 cells and more, 

 but the distance between each is considerably smaller than 

 that figured by Gran. The valves are quite flat and the 

 marginal spines are often present, although this is not always 

 the case. The odd, asymmetrical apiculus can nearly always 

 be seen. The eccentric markino^s have onlv been observed in 

 a few isolated cases, and are then usually very indistinct. In 

 one culture these markings on the valves were vei'y distinct. 



